The manufacture of lead acid batteries requires the fabrication of approximately 78 perforated lead plates or grids for each battery. One well-known method for fabricating such perforated lead grids is by gravity casting. In order to achieve proper surface characteristics, it has been known to include small amounts of antimony, calcium or other materials. The casting procedure generally involves the melting of lead pigs or bars, along with the alloying materials in a furnace or pot, and the transmission of the molten metal to a casting machine, where amounts of the molten metal are introduced between the mold portions. With such conventional techniques, several problems have arisen and remain unsolved, which problems including drossing, environmental problems in the workroom, and excessive heating costs and energy losses.
Considering each of the general problems set forth above, environmental problems include those which lead to discomfort for employees resulting from the excessive heat given off by the furnaces and the open gas-heated feed lines. The excessive amount of heat generated make conditioning of the air substantially not feasible. It would take too much make-up air in such a situation to provide any semblance of comfort within the casting room environment of prior techniques. Therefore, traditionally the casting areas for battery grids have become environmental nightmares. Operating personnel have had to endure excessive heat stress in such situations.
Further, and perhaps more importantly, excessive amounts of lead dust are normally given off both at the furnace and at the casting machines. OSHA has, at the time of this application, set limits of 0.200 milligrams of lead dust per cubic meter of air as a safe maximum allowable limit. However, it is expected that the upper limit will soon be o changed to 0.150 milligrams per cubic meter. Present methods and techniques barely meet these requirements, and if the reduction of the maximum allowable amount is made, many such systems may not be able to meet the OSHA requirements.
The aforementioned drossing is the formation of a lead oxide scum on the surface of molten lead, caused by contact of the molten lead with oxygen. Such dross cannot be cast, as the resulting product will be inferior. Therefore, at frequent periods, this dross must be removed and shipped back to the smelter for reprocessing. It cannot be remelted at the grid casting operation.
In known systems, the lead/oxygen contact could hardly be greater. The lead is subjected to oxygen because of the use of open top furnaces. Further, the more furnaces used, the greater the surface area of lead exposed to oxygen. Also, the agitation created by the pumping of the molten lead from the furnaces through the feed lines to the casting machines even further increases the amount of dross realized. Another problem is that the higher the lead is heated, the greater the likelihood of drossing. The frequent dropping of new pigs by workmen into the open top furnace further agitates the molten material, even causing some of it to splash, which not only increases the drossing, but also is quite hazardous to workmen.
The effects of the drossing are readily apparent. As drossing increases, waste increases and the production output of grids from a given amount of lead pigs decreases. Further, the frequent stoppages necessary to frequently skim off the dross from the furnaces and other places slows down the productivity of the operation. The drossing will eventually gum up feed lines to the extent that conventional feed lines do not last longer than six months. Either the gas burns up the feed line, electrically heated strip heaters will burn out, or the aforementioned drossing is pumped into the feed lines until they are completely plugged.
Another deleterious effect from the drossing occurs in the quality of the product. Excessive, even though allowable, amounts of dross in the finished grids reduce the life of batteries. Further, in addition of such alloy materials as calcium causes dross to occur considerably faster than conventional antimonal alloys so that processing lead grids with low antimony or calcium alloys is practically impossible in conventional techniques.
Very few attempts have been made to alleviate this situation and the aforementioned problems. One of them is described in U.S. Pat. No. 3,815,659 to Pavlo et al which shows the introduction of an oxygen free superheater between the conventional furnace and the ladle for boosting up the molten material to the desired casting temperature, after it is preheated to a melted, but less than adequate casting temperature in the furnace. While this may help some, the furnace is still open and the other problems still exist.